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    <title>Chapter 2.  Access Method Configuration</title>
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    <div class="chapter" lang="en" xml:lang="en">
      <div class="titlepage">
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          <div>
            <h2 class="title"><a id="am_conf"></a>Chapter 2.  Access Method Configuration </h2>
          </div>
        </div>
      </div>
      <div class="toc">
        <p>
          <b>Table of Contents</b>
        </p>
        <dl>
          <dt>
            <span class="sect1">
              <a href="am_conf.html#am_conf_intro"> What are the available access
        methods? </a>
            </span>
          </dt>
          <dd>
            <dl>
              <dt>
                <span class="sect2">
                  <a href="am_conf.html#idm140526460544896">Btree</a>
                </span>
              </dt>
              <dt>
                <span class="sect2">
                  <a href="am_conf.html#idm140526457238992">Hash</a>
                </span>
              </dt>
              <dt>
                <span class="sect2">
                  <a href="am_conf.html#idm140526457870432">Heap</a>
                </span>
              </dt>
              <dt>
                <span class="sect2">
                  <a href="am_conf.html#idm140526457479952">Queue</a>
                </span>
              </dt>
              <dt>
                <span class="sect2">
                  <a href="am_conf.html#idm140526457610544">Recno</a>
                </span>
              </dt>
            </dl>
          </dd>
          <dt>
            <span class="sect1">
              <a href="am_conf_select.html">Selecting an access method</a>
            </span>
          </dt>
          <dd>
            <dl>
              <dt>
                <span class="sect2">
                  <a href="am_conf_select.html#idm140526461794912">Btree or Heap?</a>
                </span>
              </dt>
              <dt>
                <span class="sect2">
                  <a href="am_conf_select.html#idm140526458043648">Hash or Btree?</a>
                </span>
              </dt>
              <dt>
                <span class="sect2">
                  <a href="am_conf_select.html#idm140526459252880">Queue or Recno?</a>
                </span>
              </dt>
            </dl>
          </dd>
          <dt>
            <span class="sect1">
              <a href="am_conf_logrec.html">Logical record numbers</a>
            </span>
          </dt>
          <dt>
            <span class="sect1">
              <a href="general_am_conf.html">General access method configuration</a>
            </span>
          </dt>
          <dd>
            <dl>
              <dt>
                <span class="sect2">
                  <a href="general_am_conf.html#am_conf_pagesize">Selecting a page size</a>
                </span>
              </dt>
              <dt>
                <span class="sect2">
                  <a href="general_am_conf.html#am_conf_cachesize">Selecting a cache size</a>
                </span>
              </dt>
              <dt>
                <span class="sect2">
                  <a href="general_am_conf.html#am_conf_byteorder">Selecting a byte order</a>
                </span>
              </dt>
              <dt>
                <span class="sect2">
                  <a href="general_am_conf.html#am_conf_dup">Duplicate data items</a>
                </span>
              </dt>
              <dt>
                <span class="sect2">
                  <a href="general_am_conf.html#am_conf_malloc">Non-local memory
        allocation</a>
                </span>
              </dt>
            </dl>
          </dd>
          <dt>
            <span class="sect1">
              <a href="bt_conf.html">Btree access method specific configuration</a>
            </span>
          </dt>
          <dd>
            <dl>
              <dt>
                <span class="sect2">
                  <a href="bt_conf.html#am_conf_bt_compare">Btree comparison</a>
                </span>
              </dt>
              <dt>
                <span class="sect2">
                  <a href="bt_conf.html#am_conf_bt_prefix">Btree prefix
        comparison</a>
                </span>
              </dt>
              <dt>
                <span class="sect2">
                  <a href="bt_conf.html#am_conf_bt_minkey">Minimum keys per page</a>
                </span>
              </dt>
              <dt>
                <span class="sect2">
                  <a href="bt_conf.html#am_conf_bt_recnum">Retrieving Btree records by logical record number</a>
                </span>
              </dt>
              <dt>
                <span class="sect2">
                  <a href="bt_conf.html#am_conf_bt_compress">Compression</a>
                </span>
              </dt>
            </dl>
          </dd>
          <dt>
            <span class="sect1">
              <a href="hash_conf.html">Hash access method specific
        configuration</a>
            </span>
          </dt>
          <dd>
            <dl>
              <dt>
                <span class="sect2">
                  <a href="hash_conf.html#am_conf_h_ffactor">Page fill factor</a>
                </span>
              </dt>
              <dt>
                <span class="sect2">
                  <a href="hash_conf.html#am_conf_h_hash">Specifying a database hash</a>
                </span>
              </dt>
              <dt>
                <span class="sect2">
                  <a href="hash_conf.html#am_conf_h_nelem">Hash table size</a>
                </span>
              </dt>
            </dl>
          </dd>
          <dt>
            <span class="sect1">
              <a href="heap_conf.html">Heap access method specific
        configuration</a>
            </span>
          </dt>
          <dt>
            <span class="sect1">
              <a href="rq_conf.html">Queue and Recno access method specific configuration</a>
            </span>
          </dt>
          <dd>
            <dl>
              <dt>
                <span class="sect2">
                  <a href="rq_conf.html#am_conf_recno">Managing record-based
        databases</a>
                </span>
              </dt>
              <dt>
                <span class="sect2">
                  <a href="rq_conf.html#am_conf_extentsize">Selecting a Queue extent
        size</a>
                </span>
              </dt>
              <dt>
                <span class="sect2">
                  <a href="rq_conf.html#am_conf_re_source">Flat-text backing
        files</a>
                </span>
              </dt>
              <dt>
                <span class="sect2">
                  <a href="rq_conf.html#am_conf_renumber">Logically renumbering
        records</a>
                </span>
              </dt>
            </dl>
          </dd>
        </dl>
      </div>
      <div class="sect1" lang="en" xml:lang="en">
        <div class="titlepage">
          <div>
            <div>
              <h2 class="title" style="clear: both"><a id="am_conf_intro"></a> What are the available access
        methods? </h2>
            </div>
          </div>
        </div>
        <div class="toc">
          <dl>
            <dt>
              <span class="sect2">
                <a href="am_conf.html#idm140526460544896">Btree</a>
              </span>
            </dt>
            <dt>
              <span class="sect2">
                <a href="am_conf.html#idm140526457238992">Hash</a>
              </span>
            </dt>
            <dt>
              <span class="sect2">
                <a href="am_conf.html#idm140526457870432">Heap</a>
              </span>
            </dt>
            <dt>
              <span class="sect2">
                <a href="am_conf.html#idm140526457479952">Queue</a>
              </span>
            </dt>
            <dt>
              <span class="sect2">
                <a href="am_conf.html#idm140526457610544">Recno</a>
              </span>
            </dt>
          </dl>
        </div>
        <p> 
        Berkeley DB currently offers five access methods: Btree,
        Hash, Heap, Queue and Recno.
    </p>
        <div class="sect2" lang="en" xml:lang="en">
          <div class="titlepage">
            <div>
              <div>
                <h3 class="title"><a id="idm140526460544896"></a>Btree</h3>
              </div>
            </div>
          </div>
          <p> 
            The Btree access method is an implementation of a
            sorted, balanced tree structure. Searches, insertions, and
            deletions in the tree all take
            <span class="emphasis"><em>O(height)</em></span> time, where
            <span class="emphasis"><em>height</em></span> is the number of levels in
            the Btree from the root to the leaf pages. The upper bound
            on the height is <span class="emphasis"><em>log base_b N</em></span>, where
            <span class="emphasis"><em>base_b</em></span> is the smallest number of
            keys on a page, and <span class="emphasis"><em>N</em></span> is the total
            number of keys stored. 
        </p>
          <p> 
            Inserting unordered data into a Btree can result in
            pages that are only half-full. DB makes ordered (or
            inverse ordered) insertion the best case, resulting in
            nearly full-page space utilization.
        </p>
        </div>
        <div class="sect2" lang="en" xml:lang="en">
          <div class="titlepage">
            <div>
              <div>
                <h3 class="title"><a id="idm140526457238992"></a>Hash</h3>
              </div>
            </div>
          </div>
          <p>
            The Hash access method data structure is an
            implementation of Extended Linear Hashing, as described in
            "Linear Hashing: A New Tool for File and Table
            Addressing", Witold Litwin, <span class="emphasis"><em>Proceedings of the
            6th International Conference on Very Large Databases
            (VLDB)</em></span>, 1980. 
        </p>
        </div>
        <div class="sect2" lang="en" xml:lang="en">
          <div class="titlepage">
            <div>
              <div>
                <h3 class="title"><a id="idm140526457870432"></a>Heap</h3>
              </div>
            </div>
          </div>
          <p>
            The Heap access method stores records in a heap file.
            Records are referenced solely by the page and offset at
            which they are written. Because records are written in a
            heap file, compaction is not necessary when deleting
            records, which allows for more efficient use of space than
            if Btree is in use. The Heap access method is intended for
            platforms with constrained disk space, especially if those
            systems are performing a great many record creation and
            deletions.
        </p>
        </div>
        <div class="sect2" lang="en" xml:lang="en">
          <div class="titlepage">
            <div>
              <div>
                <h3 class="title"><a id="idm140526457479952"></a>Queue</h3>
              </div>
            </div>
          </div>
          <p> 
            The Queue access method stores fixed-length records
            with logical record numbers as keys. It is designed for
            fast inserts at the tail and has a special cursor consume
            operation that deletes and returns a record from the head
            of the queue. The Queue access method uses record level
            locking.
        </p>
        </div>
        <div class="sect2" lang="en" xml:lang="en">
          <div class="titlepage">
            <div>
              <div>
                <h3 class="title"><a id="idm140526457610544"></a>Recno</h3>
              </div>
            </div>
          </div>
          <p>
            The Recno access method stores both fixed and
            variable-length records with logical record numbers as
            keys, optionally backed by a flat text (byte stream) file.
        </p>
        </div>
      </div>
    </div>
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